The present invention relates to a new method and apparatus for constructing precision net form components as well as simpler forms with precisely controlled streams of material droplets in a background gas ranging from vacuum to above atmospheric pressures where the size, energy and rate of arrival of the droplets as well as the pressure and type of background gas can all be adjusted to optimize the construction and material properties of the component.
Conventional casting consists of pouring or injecting molten metal into a mold at a rate which is faster than the solidification rate. This well known procedure is suitable for the high volume production of small simple parts with reasonably uniform dimensions. However, several deficiencies in conventional casting has lead the metallurgy industry to research new techniques of materials processing. For example, in conventional casting segregation occurs in the production of most alloys. Also, it has been found that since the solidification time for casting is long, differences in the composition of the metallic part can occur.
Powder metallurgy (P/M) is a well established production process in which parts are made by compressing metal powders in a mold. Subsequent sintering (heating) is necessary to bond the particles to give the formed material strength and other desirable properties. The powder needs to be contained and formed by dies. The advantage of powder metallurgy is that metals which are difficult to melt and to cast such as tungsten and tantalum can be economically fabricated by .the P/M process. It can also be used to produce non-metallic parts. Generally speaking, P/M involves the steps of mixing, compacting and sintering. Further steps are often taken to improve the structural soundness of the P/M part such as infiltration and repressing. Strengths of the P/M process include the ability to fabricate complex shapes, the ability of precise material control or unusual material composition, and the ability of mass production. However, due to the nature of the P/M process, it is restricted to relatively small components. Further, the cost of the powder may limit the feasibility of P/M manufacturing to a narrow range of applications.
A new method of manufacturing called net form manufacturing is currently the topic of industrial as well as academic interest. Powder metallurgy is viewed by some researchers to be a type of near net form manufacturing even though additional manufacturing processes are required to assure structural strength after the part has been formed in the mold.
Net form manufacturing refers to that process where the final, or near final engineering part is made from the raw material in one integrated operation. Subsequent working is not required to enhance the structural qualities of the net formed part. For instance, in the developing technology of spray forming, a spray of molten metal is used as the manufacturing constituent to fabricate a part in its near net form. The spray is achieved by bombarding a stream of molten metal with an atomizing or nebulizing gas. Thus, the presence of the atomizing gas in the manufacturing environment is a required (though not necessarily desirable) feature of the currently developed technique of spray forming. The spray droplets travel in the gas environment and are deposited onto a collector. Either the collector or the spray may be moved so that the deposit is constructed in the desired shape. The molten metal droplets arriving at the solidifying surface remain where they are delivered, thus there is no need for a mold. The surface consists of a thin liquid film just a few microns thick. Once the droplets impinge on the surface they "splat", as if they had impinged on a solid surface. The splatting action causes the boundaries between the surface and the drop (splat) to disappear as the fluids mix. The splat solidifies almost immediately, thus prohibiting any significant lateral migration. It has been found that the material properties of the product depends on the splatting conditions. In spray forming, the near net formed part is processed further in order to achieve the characteristics of the final finished piece. Thus, spray manufacturing is termed here as near net manufacturing. Regardless of this detail, under careful conditions, the material structure of the final form will have a finer grain than those parts conventionally cast, and will be free of macroscopic segregation. Segregation, if any, will occur on the scale of a splat diameter. The combination of low segregation and fine grain size yields a product with enhanced mechanical properties. Moreover, since there are fewer manufacturing steps than in conventional processes, the production costs can be reduced.
See "The Osprey Preform Process," Powder Metallurgy, 1985, vol. 28, no. 1, pp. 13-20 for additional information on spray forming.
While it is clear that spray forming offers significant improvement over conventional processes in certain applications, there are several deficiencies present which may be overcome by using different methods. For example, the spray of molten metal droplets is for the most part uncontrolled. The droplets within the spray cone have a wide distribution of sizes and energies which can only be described statistically. This means that the smaller droplets may arrive at the surface pre-solidified, and there would be little cohesion between the particles in the deposit, resulting in an inhomogeneous material. Also, the dimensional fidelity of the net form part is limited by the lateral extent of the conical volume of particles. Smaller intricate parts cannot be made with this method without further work. And, due to the nature of the spray process, it is inevitable that overspray will occur, and that there will be high losses from scrap. The final deficiency noted is that the deposition environment is coupled with the atomizing technique, therefore making it impossible to fabricate materials in a vacuum environment, or an environment which is independent from the atomizing gas. It is submitted that use of controlled streams of droplets that are generated without the use of an atomizing or nebulizing gas, instead of droplet sprays, will lessen if not remove the above deficiencies associated with spray forming, as well as to preserve the benefits of low cost and added strength.
It would be advantageous to have droplets arriving at the thin liquid surface with uniform and controllable size and temperature. Also, in many circumstances the background gas in the spray chamber can be trapped in the solidifying material. Thus, decoupling the size and speed of the droplets from the background gas supply provides an opportunity to optimize the droplet deposition process in order to produce the highest quality materials. An ability to have a vacuum or reduced pressure gas as the background would be advantageous in removing the problem of trapped gases or gases in solution. Finally, in some circumstances, controlled amounts of reactive gases in the background may enhance the properties of the deposited materials.
As will be described in more detail below, net form manufacturing with liquid molten metal drops is found to alleviate many of the hindrances encountered in conventional manufacturing processes, as well as to increase the structural integrity of the part. It is an object of the present invention to provide a method and apparatus for such net form manufacturing.
Recent research has lead to the precise control of droplet stream generation. Precise control refers to the ability to generate a stream of droplets with speed differences as small as 1.times.10.sup.-7 times the average droplet velocity, and angular deviations of the stream of typically a few times 1.times.10.sup.-6 radians. Further, precise control refers to the ability to manipulate the configuration of the stream of droplets by adjusting an input disturbance to the droplet generator. It has been found that the fluid stream from which droplets are formed responds to the applied disturbance almost instantaneously (on the order of one disturbance wavelength). This means that a stream of droplets can be generated which are either very uniform (1.times.10.sup.-7 times the average droplet velocity), or have a predictable and highly controllable size and spacing distribution. It is another object of the present invention to provide a method and apparatus for use of these streams in production of net forms, a process sometimes referred to as precision droplet stream manufacturing, or PDSM.
The general phenomenon of capillary stream break-up in the break-up of a liquid jet should be considered. The controlled instability of a fluid stream is introduced by disturbing the stream, as by vibrating the stream with a sinusoidal, triangular or other periodic waveform. When a fluid stream is disturbed with a disturbance, the stream breaks into a series of droplets, preferably equally spaced droplets which are separated a distance corresponding to the wavelength of the disturbance. The resulting stream of droplets is separated a distance which corresponds to the wavelength of the disturbance.
A different break-up process occurs if the stream is perturbed with an amplitude modulated disturbance. FIGS. 1a and 1b are representations of the response of the stream when perturbed with an amplitude modulated disturbance based on the present understanding of the phenomenon. The stream condition at various times t.sub.1 -t.sub.7 of FIGS. 1a is shown in FIG. 1b. A disturbance is imposed on the stream and it grows until the stream begins to break. It continues to break until the situation illustrated as t.sub.5 is reached. The droplets in this configuration are separated a distance corresponding to the wavelength of the fast or carrier frequency, and are thus termed "carrier" droplets. Unlike conventional droplets, i.e., droplets generated with a single frequency disturbance, the carrier droplets generated by the amplitude modulated disturbance have a predictable relative speed component. The carrier droplets with their corresponding relative speeds are illustrated in configuration t.sub.5 in FIG. 1b. The predictable relative speed component should not be confused with the unpredictable speed fluctuations that are measured as speed dispersions. The relative speed components are a direct consequence of the amplitude modulated disturbance waveform. That is, since the radial amplitude of the stream at the forward and rearward extremes of the potential drop are not symmetric, the break times of the extremes will be different, resulting in a net impulse, or speed change on the drop. Thus, the value of relative speed component depends on the degree of modulation of the disturbance; a highly modulated disturbance will yield a higher value and vice versa. The nature of the component is that it forces the carrier drops to coalesce systematically into larger drops as illustrated by t.sub.7 in FIG. 1b. The merging time, or the time represented by drops at t.sub.7 is always much greater than the break time of the droplets represented by t.sub.5, the time required to break into uniformly spaced carrier droplets. The merging time is predictable. The final drops are separated a distance commensurate with the wavelength of the slow or modulation frequency of the disturbance, and hence are called "modulation" drops. The modulation drops are much more uniform in spacing and have smaller speed dispersions than drops generated with a conventional single frequency disturbance. It should also be noted that the separation between droplets increases linearly with the frequency ratio N. A frequency ratio of 1 is defined here as a conventional single frequency disturbance. It has been found that as the frequency ratio increases, the velocity dispersion decreases approximately as 1/N.
See "New technique for producing highly uniform droplet streams over an extended range of disturbance wave numbers," Review of Scientific Instruments 58 (2) February, 1987, pp. 279-284, and "Applications to Space Operations of Free-Flying Control Streams of Liquid," AIAA85-1029 and the paper of the same title in Journal of Spacecraft, Vol 23, No. 4, July-August, 1986, pp. 411-419, for additional information on production of droplet streams with amplitude modulation.